There are many different sources of VOC's that have fume concentrations that vary over time. For example, certain manufacturing batch processes will have VOC's fume flows that will vary from rich to lean over an extended period (such as every 24 hours).
In addition, soils and sludges contaminated with organic chemicals are a widespread problem throughout the world, with millions of cubic meters requiring remediation in the United States alone. For example, concentrated underground organic contaminant plumes are one of the most prevalent ground water contamination sources. A typical source of concentrated plume is a leaking underground storage tank. When the stored liquid escapes from the tank slowly, it can take years for the operator to become aware of the problem. By that time, the solvent or fuel can percolate deep into the earth, often into water-bearing regions. Collecting as a separate, liquid organic phase called Non-Aqueous-Phase Liquids ("NAPL's"), these contaminants provide a source that continuously compromises surrounding ground water. This type of spill is one of the most difficult environmental problems to remediate.
Clean-up of such contaminated materials is also subject to a wide variety of regulations in the United States, including those covered under The Comprehensive Environmental Response, Compensation, and Liability Act of 1980 ("CERCLA"), The Superfund Amendments and Reauthorization Act of 1986 ("SARA"), and The Resource Conservation and Recovery Act ("RCRA"). The total cost of these clean-up efforts in the United States has been estimated to exceed $200 billion over the next 30 to 40 years.
A number of processes can be used to deal with these problems of contaminated soils and sludges. Early techniques involved simply excavating the contaminated soil and disposing of it at a facility approved for the acceptance of hazardous waste. This method has high direct costs, and can lead to serious disruption of business operations for extended periods of time.
Attempts to remove such material by pumping the ground water required that huge amounts of water be washed through the system, requiring tens of years. Pumping at some sites for many years has resulted in effluent water that was apparently clean, but when the pumps were shut off and restarted several years later, the ground water again contained contaminates because they were not fully remediated in the first place.
Processes such as thermal desorption and in-situ soil vapor extraction ("SVE") allow for the controlled separation of organics from solids and soils. In these processes, the organic material is volatilized to produce an organic vapor, which thereafter must be removed or otherwise treated. This is in contrast to direct incineration, which involves heating solid material to destruction temperatures in an oxidizing environment where volatilization and combustion of organics takes place simultaneously.